1. Field of the Invention
The present invention relates to a phosphor emitting a light in the region from yellow to red without using cadmium.
2. Description of the Prior Art
Since a phosphor material for a vacuum fluorescent display currently used which emits a light in the region of from yellow to red contains cadmium, it has been required not to use cadmium which adversely affects on environment. For this end, Y2O2S:Eu, Y2O3:Eu (red) phosphor has been proposed as a cadmium-free phosphor. However, such a cadmium-free phosphor is disadvantageous in that the luminance and lifetime performance are extremely inferior to those of the current cadmium-containing phosphor. At present, the cadmium-free phosphor which satisfies the requirements of the market has not been proposed.
On the other hand, a phosphor containing alkaline earth metal as a matrix was conventionally known. CaS:Eu and CaS:Eu, Ce phosphors emit a light in red. The luminescent color can be changed to orange color by forming a solid solution of CaS:Eu and CaS:Eu, Ce phosphors with Sr. The CaS:Ce phosphor emits a light in green and CaS:Ce, Na phosphor emits a light in blue. However, these conventional phosphors are disadvantageous in that these phosphors are easily decomposed by moisture under normal usage environment.
In a process of manufacturing a vacuum fluorescent display using the phosphor at anodes of a luminescent section of the vacuum fluorescent display, there is a step for removing an organic binder by baking the phosphor applied usually at temperatures ranging from 450xc2x0 C. to 550xc2x0 C. This results in acceleration of the surface oxidation of a phosphor layer. If the surface oxidation of the phosphor layer is accelerated, S in the components of the phosphor is separated to scatter by the impact of collision with electron beam at the time of lightning of the vacuum fluorescent display, and the filament of the fluorescent display is adversely affected.
As one of the measures to solve such problems, there has been proposed a method for improving the reliability of the vacuum fluorescent display in which the phosphor layer is formed of an organic substance which can be decomposed in an atmosphere other than air, for example, in an atmosphere of nitrogen, and baked in an atmosphere of nitrogen to prevent the surface oxidation of the phosphor to eliminate adverse effect on the filament. However, this method has disadvantages in cost and control of the process, because of complexity of the baking step.
Accordingly, an object of the present invention is to provide a phosphor for emitting a light in the region of from yellow to red in good luminous characteristic without using cadmium.
According to a first aspect of the present invention, there is provided a phosphor containing as a matrix the sulfide of a metal belonging to Group II of the periodic table which is doped with dopant to emit a visible light, wherein the phosphor contains at least one element selected from the group consisting of B, Sb, Bi and P in 0.01% to 5.0% on the basis of the weight of the phosphor.
The matrix of the phosphor may be sulfide of an element selected from the group consisting of Mg, Ca, Zn and Sr belonging to Group II of the periodic table and doped with one or two of dopant selected from the group consisting of Eu, Ce and Mn.
In an alternative embodiment of the present invention, the matrix of the phosphor may be CaSrS and doped with one or two of dopant selected from the group consisting of Eu, Ce and Mn.
According to a second aspect of the present invention, there is provided a method of preparing a phosphor containing as a matrix sulfide of a metal belonging to Group II of the periodic table which is doped with dopant to emit a visible light, wherein at least one oxide selected from the group consisting of B2O3, Sb2O3, Bi2O3 and P2O5 is added to the phosphor, mixed and baked.
According to a third aspect of the present invention, there is provided a vacuum fluorescent display comprising an enclosed container maintained under high vacuum, an anode deposited a phosphor layer thereon and a cathode for emitting electrons to be impinged upon the phosphor layer of the anode, wherein the phosphor layer is formed of a phosphor which contains as a matrix the sulfide of a metal belonging to Group II of the periodic table and is doped with dopant to emits a visible light, and is obtained by adding to the phosphor at least one element selected from the group consisting of B, Sb, Bi and P in 0.01% to 5.0% on the basis of the weight of the phosphor, mixed and baked.
These and other objects features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.